Dairy farmers on light soils may have to accept a cut in cow numbers to reduce their nitrogen leaching, says Federated Farmers president Bruce Wills.
They would have no other alternative, he said in an interview to mark the end of his three-year term.
"We're losing too much nitrogen. The massive shift to dairy has caught us, and the science hasn't kept up."
The "easy yards" of fencing waterways, modern effluent systems and fertiliser application advice had been done, but nitrogen was still leaching into streams, he said.
"It's a hard conversation, but we have to have it.
"The guys leaching 70, 80, 90kg of nitrogen per hectare per year on the lighter soils will have to get that down to 30-40kg.
"If science won't deliver the goods, we're going to have to get these people to change their farming system.
"That's not easy when a lot of them have borrowed many millions of dollars to get a system going and they've got a bit caught with interest payments."
Many farmers had changed already, Wills said. "They've read the signals, backed off from four cows per hectare to 3-3.5 cows, and put in less inputs."
This did not necessarily mean reduced profits, he said. "What we're finding is that as well as a more relaxed, comfortable farming system, they're actually making a higher net profit."
While farmers were prepared to act more responsibly to protect the environment, their businesses had to remain economic, he said. "My worry is the pendulum is going to swing too far in favour of the environment."
He said Labour finance spokesman David Parker was calling for the scrapping of the irrigation investment fund and for charges on water.
"If we had a change of government, we can kiss goodbye to any hope of meeting the ag double of increasing the export value from $32 billion to $64b."
- NZ Farmer